Of many joints in a human body, formed is a hip joint region being a cartilage portion in friction contact with a femur being a bone of lower limb below a hip, referred to as an acetabulum, in a pelvis, which is a hipbone. The acetabulum enables the pelvis and the femur to rotate relatively to each other in a certain range of angles. In some cases, the hip joint region in the human body cannot be restored due to wear of a joint area, aging of bone tissue, and accidents. This unrestorability is caused by natural loss due to upright walking, heredity cause, excessive workouts and safety accidents. In this case, pain is accompanied at regions of bones touching with each other between interconnected bones and the hip joint is damaged. An artificial hip joint device is used to replace the damaged joint for facilitating relative rotation and flexing as in its native state. In general, an artificial hip joint system includes an acetabular cup coupled to an acetabulum by insertion and a head of a femoral stem inserted inside a femur, and the head can rotate within a certain range inside the acetabular cup.
Likewise, a dual mobility acetabular cup assembly of several acetabular cup assemblies used in total hip arthroplasty (THA) utilizing an artificial hip joint device was developed in 1970's, and has been used and continuously improved up until now to provide as large range of motion as possible in a stable environment and reduced wear capability.
Basically, as shown in FIGS. 1 and 2, a dual mobility acetabular cup 400 comprises: an acetabular cup 410, a liner 420, an insert 430, and a ball head 440. The insert 430 being rotatable is coupled inside the acetabular cup 410 through the liner 420, and the ball head 440 being rotatable is coupled inside the insert 430, thereby making dual bearing. The liner 420 is fixed in an inner face of the acetabular cup 410, i.e., between the acetabular cup 410 and the insert 430.
The dual mobility acetabular cup 400 provides two articulating elements by having two bearing surfaces. Of the two bearings, one being an external bearing is disposed between the acetabular cup 410 and the insert 430 and the other being an internal bearing is disposed between the insert 430 and the ball head 440. A majority of movement occurs in the internal bearing and the external bearing moves when a normal movement range is exceeded.
In total hip arthroplasty, dislocation of an implant may occur after surgery. Such dislocation after THA conservatively happens 2 to 4 percent. The patient becomes unable to move his or her body and suffers from severe pain and the doctor is placed under pressure of performing emergency operation when dislocation occurs at work hours or night. Also, this condition diminishes life quality of the patient and may cause a medical dispute.
Innovations in the THA implant have been focused mostly on reducing an incidence of prosthetic instability over the past 20 years. As a result, larger heads, lipped liners, modularity, metal on metal THA, metal on metal resurfacing, dual mobility THA, constrained liners, smaller trunnions/neck and the like have appeared. Of these, it is known that the dual mobility acetabular cup described above reduces a possibility of occurrence of dislocation by increasing range of motion (ROM).
However, such dual mobility acetabular cup has some issues to be improved.
In the first place, intra-prosthetic dislocation inside the artificial implant requires open reduction, which is as problematic as dislocation of a head from a mobile polyethylene (PE) liner. However, this problem can be solved by using cross-linked PE of high quality.
In prior art, the liner 420 coupled inside the acetabular cup 410 protrudes toward a rim 422 and collides with a stem neck as shown in FIG. 3, which may cause micromotion and fretting corrosion. This is because the coupling of the two components is accomplished by taper locking of the rim 422.
Therefore, there is a need for developing a non-modular dual mobility acetabular cup which can improve fastening mechanism between a liner and a shell and prevent fretting corrosion.